Alistair Darling has welcomed news of a surprise jump in GDP as proof Labour's policies had been working – and insisted the coalition was wrong to claim the economy was in a worse state than generally believed before the election.

The former chancellor said GDP growth of 1.1% in the second quarter – twice the pace forecast – vindicated his economic policies.

"This is the fastest growth we have seen for over four years. It shows that confidence was returning. And you can see the success of maintaining support for important sectors like construction," he said. "And this is the final nail in the coffin of the coalition's argument that things were worse than they believed before the election."

Chancellor George Osborne said the news that the services sector and construction were driving the return to growth created a good foundation for the new government's rebalancing of the economy towards the private sector. "Today's figures show the private sector contributing all but 0.1% of the growth in the second quarter," he said.

He also said the data vindicated his plans to cut spending and hike taxes in his budget, the toughest in decades, as he seeks to cut Britain's deficit. "[They] put beyond doubt that it was right to begin acting on the deficit now," he added.

"While I am cautiously optimistic about the path for the economy, the job is not yet done. The priority now is to implement the budget policies which support rebalancing and help ensure the sustained growth that the Office for Budget Responsibility forecast this year and next."

But Darling, now Labour's shadow chancellor, said the stronger growth did not mean it was safe to push through the new government's austerity measures. "The coalition's economic policy is not inevitable – it's the choice they've made. And they will have to accept responsibility for the risks they are taking with the economy," Darling said.

Trade unions are also worried the government is cutting too soon and too deeply. TUC general secretary Brendan Barber said: "The impressive GDP figures are the result of fiscal stimulus and active policymaking. But continued growth cannot be taken for granted, and there is now a huge risk that cuts in spending will bring the recovery to a shuddering halt.

"With depressed export markets, unemployment remaining stubbornly high and consumer confidence extremely fragile as people fear for their jobs, the worst conclusion that could be drawn today is that recovery has now been secured."